THYROID DEFICIENCY AND WEIGHT GAIN
Other factors are also responsible for today's obesity epidemic. When an individual's hormones are out of balance, it may be impossible to achieve sustained weight management. Even when blood tests reveal "normal" thyroid hormone status, there is often an inability to convert T4 thyroid hormone into T3, which is necessary for natural thermogenesis (fat burning) to occur. T3 deficiency is another hormonal reason why excess weight accumulates with aging.
Additionally, if testosterone and/or estrogen levels are out of balance, excessive fat gain is often the result. (Very safe drugs and nutrients are available to correct these obesity-inducing hormone imbalances.)
As already stated, we tend to put on weight as we grow older, in part because aging impairs our ability to metabolize carbohydrates. Because most foods, besides fats, are eventually broken down into glucose (blood sugar), a decline in our ability to metabolize glucose is a significant cause of degenerative disease and the excessive weight gain associated with aging. One cause of impaired carbohydrate metabolism is subclinical thyroid deficiency.
Blood tests are not always reliable to diagnose subclinical thyroid deficiency. A study found that 14% of elderly people who were initially diagnosed as having normal thyroid levels were later found to have significant thyroid deficiency after undergoing extensive testing. Some physicians believe that most people over 40 have a subclinical thyroid deficiency that contributes to their weight gain (Bemben et al. 1994; Samuels 1998).
The thyroid gland secretes hormones involved in cellular energy expenditure. When an individual diets, the enzyme 5-monodiiodinase that is necessary to convert the thyroid hormone T4 into metabolically active T3 is reduced. T3 deficiency results in a slowing down of the body's metabolic rate and less food is metabolized to energy. This decrease in metabolic rate occurs because your body thinks you are starving and tries to conserve energy until more food is found.
Most of those who diet know about the "rebound effect"--the body resists losing weight while you "starve yourself," but then puts the weight back on with devastating speed when you begin to eat a little more. This is why dieting is such a miserable way to try to lose weight. But now you know why--it is because low T3 levels impair metabolic reactions by reducing your energy production and keeps you from losing weight. This biological mechanism involving thyroid hormone conversion, evolved over hundreds of thousands of years to counter starvation, is a sabotaging factor even when you deliberately eat less in an attempt to lose weight.
To illustrate how thyroid hormone status dictates body weight, consider the fact that when the thyroid produces too much thyroid hormone, the most common clinical symptom is significant weight loss. Hyperthyroidism is the name of the condition caused by an overactive thyroid gland. In 76-83% of cases, patients' first complaints to their physician are about how much weight they have lost.
On the other hand, clinical studies have shown that dieting produces a decline in the conversion of T4 to T3, resulting in a severe reduction in resting energy expenditure. This reduced metabolic rate prevents cells from burning calories to produce energy. If the cells do not take up glucose to produce energy, sugar is stored as fat within the body. The only way dieting can produce significant long-term weight loss is for the cells to take up glucose for conversion into energy rather than into body fat. When the flow of glucose is reversed from conversion into fat and storage in fat cells it is then necessary for the fat cells to release fatty acids for energy production. This is why thyroid hormone supplementation is so important to many people seeking weight loss through dieting. Not only does thyroid hormone replacement maintain healthy thermogenic activity, but it helps fight fatigue, depression, and other common disorders associated with calorie restriction. Individuals who have thyroid hormone deficiency should be prescribed a drug such as Cytomel or Armour (only under supervision of a physician).
While there are studies showing that thyroid supplementation promotes weight loss in some individuals, thyroid supplementation should only be used when there is evidence of a thyroid hormone imbalance due either to decreased secretion from the thyroid gland or decreased conversion of T4 to the more metabolically active T3 in the peripheral tissues. It is important to remember that as the body attempts to slow the metabolic rate to conserve body mass, many individuals become thyroid deficient in response to dieting. This is the normal response of the body to caloric restriction, a condition that prompts conservation of energy by inhibition of general metabolism.
Therefore, an individual with normal thyroid status before dieting may become thyroid deficient as a result of reduced intake of calories. For optimal fat-loss effects, an individual may require Cytomel or Armour drug therapy if they expect that eating fewer calories will result in significant long-term weight control.
In summary, thyroid hormones increase the metabolic rate of the entire body by accelerating the rate at which all energy sources are burned for energy and for heat. This is particularly important in Northern or colder climates. Thyroid hormones literally uncouple the metabolic processes that burn sugar and fat for the production of ATP, the energy currency of the body. ATP is a stored form of energy because it can later enter into reactions that transfer this energy to other bodily processes. Thyroid hormones uncouple the conservation of energy as ATP as fats and glucose are burned by allowing the released energy to be dissipated as heat rather than stored as ATP. Think of the role of thyroid hormones as conservers of energy. This function is useful to prevent eventual death due to starvation because it allows one to survive longer upon available stores of predominantly fat, while conserving glucose for those key organs that can only survive on glucose, such as the brain, kidney cortex, and red blood cells (at least initially for the brain). On the other side of the coin, release of thyroid hormones in cold environments prevents an early death due to freezing by accelerating the burning of predominantly fat (and some glucose) for energy production as heat.
The unifying principle here, and the fact most pertinent to the dieter trying to lose body fat, is that the body will almost always favor the burning of glucose over fat as long as there is a surplus of glucose. Fat is only burned when it is necessary to spare glucose. This is mainly due to the fact that glucose burns very cleanly and efficiently in all tissues, whereas, the utilization of fat requires the continual input of energy before fat can be burned. Another key fact is that fat can never be converted into glucose and the body’s ability to store glucose as glycogen is very limited. Consequently, the body can store a hundred-fold higher amount of energy as fat than as glucose or glycogen. This is why fat is conserved for times when glucose supply is threatened. The dieter that can reduce carbohydrate intake long enough so as to drop blood glucose levels will trigger many hormonal reactions to increase the burning of both fat and excess protein. This is done predominantly through the release of adrenaline, which directly releases fatty acids into the blood from fat cells, and through release of adrenal glucocorticoids that accelerate the rate of conversion of fat and protein into glucose. In time, such as during prolonged starvation, the rate of protein catabolism is slowed and the body for the most part is running on fat metabolism for energy, while the catabolism of protein continues primarily to enable the synthesis of glucose for a small set of tissues that can not survive on fatty acids or other fatty acid intermediates.
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How Dieting Effects Metabolism/Thyroid Function 
